Fissionable nuclear fuel comprises a variety of compositions and forms of fissionable materials, including ceramic compounds of uranium, plutonium and thorium. Fuel compounds for commercial energy generating reactors typically comprise oxides of uranium, plutonium and thorium, and mixtures thereof. The generally most suitable and commonly used fuel for such commercial nuclear reactors is uranium dioxide, which can be combined with minor amounts of other fuel materials and include neutron flux controlling additives such as gadolinium.
Commercially produced uranium dioxide is a fine, fairly porous powder, a form which is not suitable as such for use as fuel in commercial reactors. A number of means have been developed and used to convert powdered uranium dioxide into a form suitable for use as a fuel in power generating nuclear reactors. One commonly used technique has been to sinter appropriately sized bodies of the powdered uranium dioxide material at high temperatures to develop strong diffusion bonds between the individual powder particles.
However, the sintering technique requires a preliminary compressing of the loose powder into a shaped, and self-retaining compacted body of particles of sufficient strength and integrity to survive handling and the sintering procedure. The operation of compressing fine particles into a body or coherent compact with acceptable low reject levels, and with the strength and uniformity for enduring subsequent handling and firing has been a subject of considerable concern and investigation in the nuclear fuel industry.
Conventional organic or plastic binders commonly used in powder fabrication have been considered to be unsuitable in nuclear fuel processing operations. Entrainment of any binder residues such as carbon within the sintered nuclear fuel product is unacceptable in reactor service. Moreover, the presence of any organic binder among the particles inhibits the formation during sintering of strong diffusion bonds between the particles, and adversely affects the density of the sintered product. The complete removal of binders, or their decomposition products, prior to sintering is especially difficult, and usually requires a costly additional operation in the fuel manufacture.
Accordingly, a common method has been to die press uranium dioxide powder into appropriately sized "green" (unfired) compacts without the assistance of any binder. This approach however has resulted in very costly high rates of rejects and scrap material recycling because of the weakness of such green, binder-free compacts of powder.
U.S. Pat. No. 4,061,700, issued Dec. 6, 1977, to Gallivan, and assigned to the same assignee as this application, discloses a distinctive group of fugitive binders that improved the production of sintered pellets of particulate nuclear fuel materials for nuclear reactors. The fugitive binders of this patent function without contaminating the resulting fuel product, and they permit the formation of effective bonds between sintered particles during firing without deleteriously affecting the desired porosity of the fused pellet.
The disclosure of the said U.S. Pat. No. 4,061,700, and of U.S. Pat. Nos. 3,803,273; 3,923,933; and 3,927,154, assigned to the same assignee as the instant application, and relating to significant aspects in the subject field of producing nuclear fuel pellets from particulate fissionable ceramic material for reactor service, are all incorporated herein by reference.
The prior art techniques or means such as disclosed in U.S. Pat. No. 4,061,700, have been found wanting in some conditions and circumstances. For instance it has been observed that the fugitive binders of the aforesaid patent do not provide consistent results as to pellet strength and integrity irrespective of the blending conditions and particle characteristics of the uranium dioxide powder. Specifically the severity of agitation in blending, relative humidity and temperature, and duration of storage, as well as the uranium oxide powder properties such as size, surface area and moisture content are all factors that apparently can detract from the uniformity of the physical attributes provided by such fugitive binders.
This invention deals with the typical brittle nature of ceramic materials and problems imposed thereby when compression molding such materials comprising uranium dioxide powder and also occurring in the resulting molded compact. As is well known, ceramic materials are generally of a relatively brittle consistency as opposed to a plastic or conformable consistency. Thus, rather than gradually deforming over a period of progressively increasing applied compressive stress approaching the breaking point as is the case with a plastic material, ceramics tend to rigidly resist substantially all deformation until the breaking point is reached whereupon they abruptly fracture with the resulting fissure or fissures instantly progressing through the mass fragmenting it. An apt illustration of this brittle and unyielding property and the fracture characteristics of a ceramic is the crushing of a glass marble. On the other hand, a plastic material will gradually yield and deform with progressively increasing compressive stress until reaching its breaking point and rupturing, and commonly the propagation of the resulting fracture is of a slower rate and does not continue to the extent of fragmenting the mass. Thus a plastic type of material is more amenable to compression molding than the brittle type of materials.
This inherent brittle characteristic in uranium dioxide powder, or its lack of plasticity, constitutes a significant shortcoming when subjected to compression molding operations and in the properties of the molded products.